Complex answer as MQX4.2 is last Classic version (i.e. standalone) and will only have maintenance updates for bug fixes and not be adding in new features and/or new devices. It will remain supported for 1 years+.

KDS_3.0/KSDK_1.3 is pretty good and has options for using MQX that is in the KSDK distribution. This MQX uses the KSDK peripheral drivers and not the same MQX drivers that are in MQX4.2 so a port is an effort.

KSDK_2.0 will be a big change early next year and will not have MQX in it. Freescale is migrating RTOS efforts to FreeRTOS.

Assuming your device remains K70 then remaining with your current tool environment is best as K70 is not supported in the KSDK as it is a legacy Kinetis device.